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MEMS and microelectrode technologies for hearing restoration

Two common types of hearing loss suffered by millions of people worldwide are sensorineural hearing loss and conductive hearing loss. The former is caused primarily by damage to the cochlea, the snail-shell like structure of the inner ear containing hair cells, the movement of which is interpreted by the brain as sound. On the other hand, conductive hearing loss relates to problems with conducting sound waves along the route between the outer ear to the middle ear, and may occur along with sensorineural hearing loss. This article examines MEMS based approaches to improving the ability to hear.

Since the development of the ear trumpet (Figure 1), hearing aids have and continue to provide help to people affected with hearing loss. The drawbacks of conventional hearing aids, in addition to their visibility (and perceived social stigma), include technology limitations such as acoustical feedback and distortion.

Figure 1: Early hearing aids.

Addressing sensorineural hearing loss by using cochlear implants

Cochlear implants incorporating MEMS and microelectronic technologies are now being marketed and continually developed to restore hearing to sufferers of severe hearing loss. The objective is to stimulate the cochlea and recreate, to the extent possible, a return to normal sound perception. The major advantage of these devices is their ability to restore near-normal hearing as compared to conventional hearing aids. The long term objective is to minimize or eliminate external components while further improving sound perception by wearers of the devices. It is estimated that some 120,000 people have now received cochlear implants worldwide.

Cochlear implants defined

Components of a cochlear implant include an outside microphone/transmitter, usually worn behind the ear, that that processes the sound into digital information. This is sent to a receiver that has been surgically implanted under the scalp. Data is sent via an electrode array to electrodes placed in an implant inserted into the cochlea. The signals are picked up by the auditory nerve and sent to the auditory center of the brain, which recognizes them as sound. Figure 2 depicts components of a conventional cochlear implant system.

In the diagram above, the sound processor (1) captures sound and converts it into digital code and transmits the digitally-coded sound through the coil (2) to the implant (3) that converts the digitally-coded sound to electrical impulses and sends them along the electrode array which is positioned in the cochlea. The implant's electrodes stimulate the cochlea’s hearing nerve (4) which then sends the impulses to the brain where they are interpreted as sound. Hearing is managed via the remote assistant (5) or directly from the sound processor.

Challenges

“The main challenges in improving these devices focus on the electrode arrays, which are normally manufactured individually,” says Jianbai Wang, a former research assistant at University of Michigan, “and the space limitations within the scala tympanic canal in the cochlea. The scala tympanic canal tapers from a diameter of 1 mm to about 200 µm over its full 35 mm canal depth. This implies,” she explains, “that only limited stimulating sites (16 to 22) may be accommodated and that the insertion depth is confined to the first few millimeters. These constraints fundamentally limit the frequency range of present prosthesis.”

Along with sizing constraints, it is also a challenge to insert the arrays into the cochlea without damaging the arrays or cochlea. Addressing this, Wang, along with her research program leader Professor Ken Wise, and others are looking at batch fabricated thin film electrode arrays in the NSF Engineering Research Center for Wireless Integrated MicroSystems at Michigan. “These promise increasing stimulation sites to as many as 128 over the same implant depth or over the full length of the canal,” says Professor Wise. “Eight piezoresistive sensors are formed with a layer of polysilicon and its insulating dielectrics, and these sensors are used to measure array-wall contact and array position in real time. This helps minimize insertion damage and optimize array placement thus insuring improved frequency range and efficient stimulation.”

“These sensors allow an overall array shape recovery with tip position accuracy better than 50 µm,” says Wang. The position sensors have effective gauge factors of 10-20. As position detection and wall-contact sensing improves so will the depth of cochlear implant and the overall hearing experience by the patient (Figure 3).

Figure 3: The reference-sensor bridge for contact sensing at the tip, and the first sensing segment integrating a curvature sensor bridge and an electrode site.

Operational Details

As described by Wang, the system’s 2.4mm x 2.4mm signal-processing chip operates from 3 volts. Among its functions are command validation, stimulus generation, sensor selection, 5-bit offset compensation, and signal conditioning modules. “Two options are available to provide power for the signal processing chip,” she says. “One is to use an implantable battery, and the other is to use an inductive RF link to transmit/receive power.

“This cochlear implant is integrated with an external sound processor, which has a microphone to collect the sound vibration,” Wang explains. “The sound vibration is converted to electrical signals that are transmitted through scalp and received by the implanted part of the cochlear implant using an inductive RF link.” System components are shown in Figures 4 and 5.

The device is inserted into the scala tympanic canal through a round window close to the base of the cochlea. In the future devices, the front-end signal processing chip will be located outside the cochlea while the remote microcontroller package will be situated underneath the skin on the back of the ear.

An approach to conductive hearing loss

Envoy Medical Corporation is marketing its fully implantable device to restore hearing to patients suffering from conductive hearing loss due to the interruption of normal sound transmission through the outer and/or middle ear. According to Patrick Spearman, Envoy’s CEO, “Esteem is not a hearing aid. Instead it uses your own eardrum as a natural microphone, picking up sounds through the ear canal, thereby using the body’s natural anatomy to reduce the background noise, distortion, and acoustic feedback that people experience with conventional hearing aids.”

It consists of three components fully implanted in the middle ear. These are the sound processor implanted behind the outer ear and two transducers called the sensor and driver both implanted in the middle ear. All materials, the company says, have been proven safe and reliable in millions of pacemakers and other implanted medical devices.

The sound processor (Figure 6a) is implanted surgically behind the ear and is programmed by the patient’s health care professional to customize settings to his or her particular hearing needs.

The sensor (Figure 6b) is attached to the ossicular chain. It picks up vibrations from the eardrum, malleus and incus bones and converts the vibrations into electrical signals. These signals are sent to the sound processor. The sound processor filters and increases the electrical signals and sends them to the driver (Figure 6c) which is attached to the stapes in the middle ear. The driver converts the electrical signals that it has received from the sound processor back into mechanical vibrations and transmits these signals to the stapes and the cochlea.

An external component, the personal programmer, allows users to adjust the system to their own comfort level or turn it off.

“The system is powered by a maintenance-free battery that lasts 4.5 to 9 years, depending on use,” says Spearman. “Once it is depleted, the battery is replaced in a minor outpatient surgical operation.”

The company says the cost of the device itself, the implant procedure (including the cost of the implanting surgeon and his or her staff, the surgical facility, and the administration of general anesthesia), and the necessary return visits for turning on and programming the device will cost approximately $30,000.

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This article is a part of MEMS Investor Journal's ongoing market research project in the area of MEMS and non-MEMS implantable technologies for biomedical applications. If you would like to receive our comprehensive market research report on this topic, please contact John Williamson at jwilliamson@memsinvestorjournal.com for more information about rates and report contents.